1. Field of the Invention
The present invention relates to a laser Doppler velocimeter for measuring, for example, the velocity of natural particulate in fluid by using a laser, thereby measuring the velocity of the fluid to be measured.
2. Description of the Related Art
As prior art, "a hot wire velocimeter", "a Pitot tube", etc., have been used as an apparatus for measuring an air velocity in a clean room, or measuring the velocity of cold air or hot air to be fed from an air conditioner, etc. Recently, a laser Doppler velocimeter having an He-Ne gas laser, a semiconductor laser, etc., as a light source is employed.
Schematic configuration of one example of conventional laser Doppler velocimeter using the above-described laser light will be shown in FIGS. 12 to 14. In FIG. 12 showing a conventional laser Doppler velocimeter, an He-Ne gas laser 1 of a light source emits a laser light for example, having a wavelength of 632.8 nm. Optical systems are disposed on the optical axis of the gas laser (GL) 1. More specifically, a beam splitter (BS) 2 splits the laser light of the gas laser 1 into two parallel laser lights. A convex lens 3 focuses the two laser lights from the beam splitter 2 to form the interference fringes of the laser lights on a region 4 to be measured. Fluid 5 to be measured is, for example, the air fed from an air conditioner, flowing to the region 4 to be measured. A convex lens 6 is opposed to a convex lens 3 to be measured to focus scattered lights reflected from the natural particulate 7 passing the region 4 to be measured at the same velocity as the air jet. A photomultiplier tube 8 used as the signal detector, receives the light collected by the convex lens 6, and outputs a signal in relation to the light into a signal processor 9. The signal processor 9 calculates on the basis of the output signal from the photomultiplier tube (PH) 8.
The signal processor 9 is an apparatus for calculating the velocity of the fluid 5 to be measured, and constituted as below. The signal processor 9 has a high pass filter (HP) 10 for removing the dc component of the output of the photomultiplier tube 8, a comparator (CM) 11 for converting the output of the high pass filter 10 into a square wave according to the threshold value of a predetermined level, a pulse counter (PC) 12 for counting the number of pulses of the wave of the output of the comparator 11, and a computing circuit (CL) 13 for calculating the velocity of the fluid 5 to be measured from the counted value of the pulse counter 12.
In the conventional laser Doppler velocimeter arranged as described above, "interference fringes" are formed on the region 4 measuring of the two laser lights of the same phase in equal intensity of the gas laser 1, split and refracted by the beam splitter 2. The natural particulate 7 existing in the fluid 5 to be measured pass the region 4 to be measured, i.e., the interference fringes at the same velocity by feeding the fluid 5 to be measured to the region 4 to be measured. When passing the interference fringes, the natural particulate 7 emits scattered light varying strongly and weakly in response to the interference fringes. A portion of the scattered lights is condensed by the convex lens 6, and the condensed light is converged and amplified to a signal responsive to the scattered lights varying strongly and weakly by the photomultiplier tube 8, i.e., a Doppler signal, and the signal is output to the signal processor 9.
In the signal processor 9, the Doppler signal is converted to an ac voltage signal having an amplitude at 0V as a center by removing the dc component from the component of the signal input to the high pass filter 10, and the variation of the reflected light generated when the natural particulate 7 passes the interference fringes is output. The output of the high pass filter 10 is compared with the threshold value of the predetermined value by the comparator 11, converted to a rectangular wave, and the number n of the rectangular waves per unit time is obtained by the pulse counter 12. The velocity v in a direction perpendicular to the interference fringes of the natural particulate 7 is calculated by the following equation from the spacing d of the known interference fringes, the counting time t of the pulse counter 12 and the counted value N (the number of the square waves) by the computing circuit 13. EQU v=n.times.d/t
Since the velocity v calculated by the above equation is also that of the fluid 5 to be measured which is at the same velocity as the natural particulate 7, the velocity of the fluid 5 to be measured can be measured by previously disposing the region 4 to be measured so that the interference fringes become perpendicular to the flowing direction of the fluid 5 to be measured.
However, in the above-described prior art, if particles which are larger than the interval of the interference fringes constructed in the fluid 5 to be measured pass the interference fringes, or if a natural particulate 7 having larger size than the fringe show passes the interference fringes, the Doppler signal output from the photomultiplier tube 8 becomes a waveform 14 containing a dc component as shown in FIG. 13A. However, when the Doppler signal passes the high pass filter 10, it is converted to the waveform 15 of the ac voltage signal at 0V as a center as shown in FIG. 13B, and fed to the comparator 11. Thus, if the particulate 7 to be measured, having smaller size than the shoring of the interference fringes pass the interference fringes, the Doppler signal of the waveform 16 in FIG. 14A to be output from the photomultiplier tube 8 cannot be distinguished from the AC signal of the waveform 17 shown in FIG. 14B showing the state after passing the high pass filter 10. Therefore, since it measures including inaccurate data when large particles of low velocity or a plurality of natural particulate 7 pas the interference fringes, there arises a problem that the error of the measured results is increased.
Since the entire apparatus is increased in size by disposing the region 4 to be measured so that the flowing direction of the fluid 5 to be measured becomes perpendicular to the interference fringes or by regulating the optical axis of the optical system by using the photomultiplier tube 8 for photoelectric conversion, the apparatus is not easily handled, and mobility in the case of measuring mobility is lacking.
In order to eliminate the disadvantages, it is possible to use a semiconductor photodetector of small size, for example, "an avalanche photodiode" instead of the photomultiplier tube 8. However, if the natural particulate 7 in the fluid 5 to be measured passing the interference fringes such as, for example, the normal flow of the air is measured, the particles contained in the air in natural state have 0.5 to 1 .mu.m of a diameter being small. Therefore, the intensity of the scattered light reflected from the air is reduced. Thus, the obtained S/N ratio is very deteriorated. Since the the avalanche photodiode which receives the reflected light has an amplification factor of a signal of 1/1000 or less than that of the photomultiplier tube 8, its output signal has an improper S/N ratio. As a result, signal processor 9 may not operate properly, and there is a danger of calculating un erroneous velocity.
In order to improve the S/N ratio of the prior art as described above, it was necessary to measure the intensity of the reflected light from the natural particulate 7 by improving it by increasing the intensity of the laser light reflected from the natural particulate 7 by using a laser having a large output as the light source. However, this configuration increases the size of the entire apparatus. As a result, its portability is reduced. It is also difficult to obtain a simple and ready measurement of velocity.
On the other hand, a method of feeding particles having a large diameter into the fluid 5 to be measured and measuring strong reflected light from the particles can be considered. However, the more the particles are increased in size, the greater the intensity of the reflected light from the particles becomes. However, a problem exists in that large particles flow at a lower velocity than the fluid 5 to be measured.
As a result, the velocity of relatively large particles flowing at a lower velocity than the fluid in the fluid to be measured will be obtained and inaccurate data will be generated if a plurality of natural particulate pass substantially simultaneously through the interference fringes. Therefore, there arises a problem in that the error of the measured result is also increased.